Add FUSE_FILL_DIR_DEFAULTS enum (#903)

Add FUSE_FILL_DIR_DEFAULTS enum (#903)

In order to use the fuse_fill_dir_t function in a C++ program, add the enum item:
FUSE_FILL_DIR_DEFAULTS

Without this change g++ compilation failed with
example/hello.c:94:35: error: invalid conversion from ‘int’ to ‘fuse_fill_dir_flags’ [-fpermissive]
   94 |         filler(buf, ".", NULL, 0, 0);
      |                                   ^
      |                                   |
      |                                   int